Endocrine Abstracts

Background: Conservation of genes involved in TH transport, metabolism and action can reveal clues about the origins of TH signaling. A deiodinase homologue (DdDio) was previously identified in the social amoeba Dictyostelium discoideum (Singh, 2014). Dictyostelium cells live as single cell amoebae in soil where they feed on bacteria. However, upon starvation a developmental program is initiated that results in the formation of a multicellular fruiting body consisting of a stalk of dead cells that supports a sporehead in which 80% of the cell population survives as spores. DdDio knock-out cells are disturbed in aggregation and developmental gene expression, indicating a potential role for DdDio in development. In this study we determined whether Dictyostelium has deiodinating activity towards iodothyronines.

Methods: Lysates from Dictyostelium cells in the early culmination phase were prepared and measured for deiodinase activity using several radiolabeled iodothyronines as substrate.

Results: We found limited iodine release with T3 and T4, but efficient outer ring deiodination of rT3, 3,3’-T2 and 3’,5’-T2. We did not detect any inner ring deiodination. For rT3 as substrate, we found a Vmax of 1.2 (± 0.1) pmol/min.mg total lysate protein, a Km of 21 (± 2) nM, and an IC50 of 19 (± 0.5) nM in cis-inhibition studies. The Dictyostelium deiodinase activity is dependent on DTT as a co-factor and fully inhibited by 0.1 mM iopanoic acid, but not by 0.1 mM PTU.

Conclusion: Our results show that the Dictyostelium deiodinase has high affinity and activity towards certain iodothyronines. To our knowledge, this is the most distant species from humans in which iodothyronine deiodination has been found to date, pushing the root of iodothyronine metabolism back to at least 1 billion years. Future studies will have to reveal whether iodothyronines are produced and have signaling functions in Dictyostelium. Singh et al. Dev Biol 2014, 396: 256-68